Proper configurations of controls, sensors, and metrology technologies have enabled precision machines to achieve nanometer positioning. However, at this level of positioning resolution, vibration sources can become a limiting factor. One of the largest sources of vibration in, for example, a precision turning machine is an unbalanced rotating spindle, creating a rotating force vector with a once per revolution period. The cause and size of such an unbalance force is typically a function of the spindle, the part, the part fixturing, the part setup, and the spindle speed. For example, the magnitude of the unbalance force increases as the square of the spindle speed. In addition, certain spindle speeds coupled with the size of the unbalance force can contain other harmonics that may excite machine structural resonances.
For example, spindle unbalance forces in a precision machine can impart energy into the machine base and provide a forcing function to the machine slides and the machine metrology frame, and may cause undesirable slide motion. This motion can be rejected to some extent by the control system loop gain but the loop gain decreases with increasing frequency. This is the opposite of what is desired as the unbalance spindle forces increase with spindle speed (frequency) to further compound the problem. Force disturbance of the metrology frame can cause non-rigid body motion of the frame and distort the measure tool position. Canceling or otherwise attenuating the spindle unbalance force reduces these error sources.
It is also appreciated that synchronous demodulation is a well known technique to recover a synchronous signal in the presence of noise or non-synchronous interfering signals. It is in essence a process that results in a high Q filter. The desired signal is modulated onto a carrier signal where it can be signal conditioned. After conditioning, the carrier is then demodulated by a phase sensitive detector and sent to a low pass filter to recover the desired conditioned signal. Synchronous demodulation works because the input carrier signal is synchronous to a reference signal applied to a phase sensitive detector. Because the detector is sensitive to phase, the low pass filter output can be bipolar. The detector output follows the equation: A[cosine (phase of input signal—phase of the detector reference signal)], where A is the amplitude of the detector input. The detector gain is a maximum when the phase of the input signal and the reference signal are the same or 180 degrees out of phase. And the detector gain is a minimum when the phase difference between the input signal and the reference signal is 90 degrees.
Ideally, the solution is to cancel the unbalance force at the source or rotor of the spindle. This is difficult, however, since the rotor must hold the part and any apparatus that would be used to cancel the rotor unbalance. There is therefore a need for an method and system for reducing the spindle housing vibration in a feedback process utilizing synchronous demodulation using the inherent commutation functions available on a typical servo controller.